The energy content of a battery is proportional to its operating voltage, and high voltage may be achieved by utilizing anode materials which are oxidized and reduced at very negative potentials. Currently high energy batteries contain anodes comprised of alkali metals, alkali metal alloys and/or lithium intercalated carbon. These high voltage anodes typically operate outside of the thermodynamic window of their respective electrolytes and only function through the formation of a passive film which permits cation transport but blocks electron transport thereby inhibiting the self-discharge of the battery. Although essential for battery operation, these passive films also limit charge/discharge rates and contribute to irreversible losses in the cells which, in turn limit, cycle life.
These issues can be resolved, in part, by using a liquid alkali metal anode (e.g., molten sodium) and a solid electrolyte as demonstrated with the “zebra” cell technology. However, these cells operate at high temperature.
It is also recognized that unlimited cycle life can be obtained by using solvated transition metal anode and cathode materials as demonstrated by the all-vanadium redox flow battery system. However, the high solvent content required to solvate the electroactive vanadium and the low voltage of the V+2/V+3 redox reaction yield a very low energy density despite the long cycle life.
Recently Yazami (U.S. published application 2010/0141211 and Tan, Grimsdale, Yazami, J. Phys. Chem. B, 116 (2012) p. 9056) proposed that polyaromatic hydrocarbons could be reduced with Li metal to form a solvated electron, and the solvated electron is stabilized as the ion pair Li+(polyarene•−), which could serve as a “liquid lithium anode.” In effect, energy is stored at the anode as a soluble anion radical. This concept may enable virtually unlimited cycle life at very high negative potential, however, the energy density is still limited by the high electrolyte content needed to solvate the radical anion. The capacity of Yazami's highest conductivity composition (Li1.0β1.0(THF)8.2) is 36 mAHr/g (β=Biphenyl).
Separators are required for the organic solvent mediated redox flow battery to prevent the intermixing of the redox mediators in the electrochemical stack. However, these separators should be highly stable, and typically more stable than separators that are used in aqueous systems because the voltage drop across the separator is significantly larger and the organic electrolytes are typically more aggressive against conventional polymers used for aqueous separators.